For eg amoeba is a unicellular organiam
amoeba takes in food using temporary finger like extantion of the cell surface which fuse over the food particles forming a food vacuole.
complex substances are breaken down into simpler one which then diffuse into  the cytoplasm. the remaing undigested materials is moved to the surface of the cell and thrown out.

Amoeba-an unicellular organism takes in food from any point on the body surface by the formation of finger like processes called pseudopodia this process is called phagocytosis(cell eating) in hydra finger like processes around the mouth called tentacles help in ingestion of food